By software accounts, you mean those like www-data and ftp right?
–
steveyangJan 18 '12 at 5:57

Those are good examples of software accounts, yes. Also ntp, or postgres, postfix.
–
Daniel PittmanJan 18 '12 at 6:00

Thx. Those accounts in my system are mainly setup by software itself. Is there conditions I need to setup such accounts manually?
–
steveyangJan 18 '12 at 6:04

Probably not terribly often, no. But if you were compiling the software from scratch yourself, it's easy to imagine a need as part of make install. Since there's not a tradition in *NIX of hiding the options used by scripts like the one run by make install from users (even in situations where it's rare for a user to want to use such an option, like mysql's -B flag), the option is available for you to use interactively on the off chance you'd ever need it.
–
BMDanFeb 2 '12 at 17:36